Hahaha I wish this was in my life like 4 days ago. I spent a long time making this: https://anvil.works/build#clone:V6M2LVQKVGJLBKVJ=PCRIAAOHWFJFNWNQR5BE2DNI
I am not as skilled at the javascript stuff, so my code is hacky in places, but it works relatively well. The multiselect is a combination of the token box implementation plus the popovers. I had to modify the popovers code in a couple of places to help with some of the resizing and show/hide functions.